Re: Proposal: Conflict log history table for Logical Replication

On Wed, May 6, 2026 at 4:55 PM vignesh C <vignesh21(at)gmail(dot)com> wrote:
>
> On Fri, 1 May 2026 at 19:16, Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > On Thu, Apr 30, 2026 at 10:40 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > >
> > > On Wed, Apr 29, 2026 at 12:34 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> > > >
> > > > On Wed, Apr 29, 2026 at 11:50 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > > > >
> > > > > On Tue, Apr 28, 2026 at 7:53 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > > > > > > 2.
> > > > > > > +typedef enum ConflictLogDest
> > > > > > > +{
> > > > > > > + /* Log conflicts to the server logs */
> > > > > > > + CONFLICT_LOG_DEST_LOG = 1 << 0, /* 0x01 */
> > > > > > > +
> > > > > > > + /* Log conflicts to an internally managed conflict log table */
> > > > > > > + CONFLICT_LOG_DEST_TABLE = 1 << 1, /* 0x02 */
> > > > > > > +
> > > > > > > + /* Convenience bitmask for all supported destinations */
> > > > > > > + CONFLICT_LOG_DEST_ALL = (CONFLICT_LOG_DEST_LOG | CONFLICT_LOG_DEST_TABLE)
> > > > > > > +} ConflictLogDest;
> > > > > > > +
> > > > > > > +/*
> > > > > > > + * Array mapping for converting internal enum to string.
> > > > > > > + */
> > > > > > > +static const char *const ConflictLogDestNames[] = {
> > > > > > > + [CONFLICT_LOG_DEST_LOG] = "log",
> > > > > > > + [CONFLICT_LOG_DEST_TABLE] = "table",
> > > > > > > + [CONFLICT_LOG_DEST_ALL] = "all"
> > > > > > > +};
> > > > > > >
> > > > > > > Defining an array this way could be an Array size issue. Actually the
> > > > > > > array has just three elements so the last element should be at
> > > > > > > ConflictLogDestNames[2] but if we go by the above definition, it will
> > > > > > > be ConflictLogDestNames[3]. Can we define by referring the following
> > > > > > > existing way:
> > > > >
> > > > > I was analyzing this because I remember we were initially using the
> > > > > format you suggested and switched to the bit format to enable direct
> > > > > bitwise operations elsewhere. I think Peter suggested that [1], and
> > > > > the argument was that the bitwise operation is easy if we represent
> > > > > them as a bit. Also, since we would not have too many options, the
> > > > > array size shouldn't be an issue. But I understand your point: adding
> > > > > more elements will cause the array size to grow very fast as this is
> > > > > using sparse array. Let's see what others think about this, and then
> > > > > we can decide whether to change it back?
> > > > >
> > > >
> > > > The benefit of the current approach is that checking whether the
> > > > destination is TABLE becomes straightforward:
> > > >
> > > > IsSet(opts.conflictlogdest,CONFLICT_LOG_DEST_TABLE)
> > > >
> > > > if we go by regular enum values (simialr to XLogSource), then it will be:
> > > >
> > > > if (opts.logdest == CONFLICT_LOG_DEST_TABLE ||
> > > > opts.logdest == CONFLICT_LOG_DEST_ALL)
> > >
> > > Right
> > >
> > > > For ease of extending the enum and its corresponding text mappings, my
> > > > personal preference is still the regular (non-bitwise) enum approach.
> > >
> > > Yeah, that's my personal preference too. But Peter had strong stand
> > > on keeping as bitwise so that we can directly use
> > > IsSet(opts.conflictLogDest, CONFLICT_LOG_DEST_TABLE) operations.
> > > Since this array shouldn't have many options, a sparse array is not an
> > > issue. So lets see what @Peter Smith has to say here and then we can
> > > build a concensus on this.
> > >
> > > > But if we anticipate adding more destination options in the future
> > > > that would be covered by ALL, checking for those in code could lead to
> > > > growing chains of OR conditions, whereas the bitwise approach scales
> > > > more cleanly in that respect. So I think the choice depends on what
> > > > kinds of future extensions we expect.
> > > >
> > > > Do we have plans to add more options that would naturally fall under
> > > > ALL? Or do we instead expect additions that are mutually exclusive;
> > > > for example, splitting CONFLICT_LOG_DEST_LOG into something like
> > > > CONFLICT_LOG_DEST_JSON_LOG and CONFLICT_LOG_DEST_TEXT_LOG, which may
> > > > not make sense to group under ALL in the same way?
> > >
> > > Currently, I haven't considered which options would naturally fall
> > > under "ALL." Perhaps if we plan targets other than logs and files,
> > > those might also fall under "ALL."
> >
> > I have fixed all the reported comments except these four.
>
> Few minor comments:
> 1) Now that we create the table in pg_conflict system schema where
> other users cannot create the table, is there a scenario where this is
> possible?
> /*
> * Check for an existing table with the sname name in the
> pg_conflict namespace.
> * A collision should not occur under normal operation, but we
> must handle cases
> * where a table has been created manually.
> */
> if (OidIsValid(get_relname_relid(relname, PG_CONFLICT_NAMESPACE)))
> ereport(ERROR,
> (errcode(ERRCODE_DUPLICATE_TABLE),
> errmsg("conflict log table pg_conflict.\"%s\" already
> exists", relname),
> errhint("A table with the same name already exists. "
> "To proceed, drop the existing table and retry.")));
>

It is possible to hit it with allow_system_table_mods=on. See issue1
raised by Nisha in [1]

[1]: https://www.postgresql.org/message-id/CABdArM6jpLnzC5O%3DX48RpFXRmAr5WOSHJtw0ebT%2B7Wmb-WdfvQ%40mail.gmail.com

thanks
Shveta